It's simple pregnancy math.
When Chicago Fire began its second season last week, Lieutenant Kelly Severide, played by Taylor Kinney, was presented with some eyebrow-raising numbers.
Kelly was told by a friend that the due date for the woman who is having what's alleged to be his baby doesn't quite add up, given the time frame when the baby supposedly was conceived. And in typical male fashion, Kelly's first reaction was annoyed confusion. He lashed out at the messenger.
"As a guy, even though you can do the math with nine months and all that, a lot of guys will let that blow over," says a chuckling Kinney, who in real life is dating Lady Gaga.
"I was asking other guys and people with kids. I feel like some women could get away with saying just some ridiculous date. And for another woman (in this case Leslie Shay, played by Lauren German) to do the math, sit Kelly down and lay it out, he's still kind of wary of where she's going and defending whatever stance he has.
"It's just a testament to the caveman inside of us. It could be like a three-month difference, and some guy would say, 'So, what are you getting at?' "
Luckily for Kinney's Kelly character in Chicago Fire, which airs Tuesdays on NBC and Global, there generally is more firefighting than math. But there's no doubt Kelly carries a lot of the dramatic load in the show, whereas some of the other firefighters just seem to be yukking it up.
"Any time I try to liven it up and do something (funny), it gets cut anyway," Kinney says. "I have a lot of scenes with Lauren German's character and when we're waiting to shoot, we goof around, and sometimes it translates into what we do on screen. So we'll start and try something that's a little off the wall. We always think it's hilarious, but it never makes the cut.
"Having a show executive produced by Dick Wolf, you assume it's going to have these really heavy story lines. But I think what's really great about Chicago Fire is the comic relief, despite whatever brooding seriousness (Kelly) has going on."
Kinney obviously knows all about celebrity culture through his girlfriend Lady Gaga. But despite his previous roles on various TV shows, most notably The Vampire Diaries and Trauma, Kinney has found that he personally is far more recognizable since Chicago Fire debuted in 2012.
"I was in a big film a year ago (Zero Dark Thirty), I did the awards circuit and it was well known, but I rarely if ever get stopped for that," Kinney says. "If you're walking down the street and you have somebody say they watch (Chicago Fire) and they enjoy it, it's reassuring momentarily, because you feel as if you have some sense of job security, even though there's not any."
As long as continued employment for Taylor Kinney's Kelly Severide doesn't come down to a math exam, his fire will keep burning.